Lecture 14 Flashcards
describe endocytosis pathway - specific
- Vesicle traffic between PM, early endosome and trans-Golgi
- Early endosome matures into multivesicular body (MVB) and late endosome= Membrane switches
from Rab5 to Rab7 - Late endosome matures into lysosome
- Other vesicles traffic to lysosome
describe endocytosis pathway - gen
clathrin coated vesicles - no specificity with cargo
like cholesterol most have receptors
early endosome –> late endosome –> endolysosome –> lysosome
all components degraded - macromolecules to monomers
describe endosome tethers
complexes have same core subunits
different end subunits bind diff rabs
also bind snares
both recognize hops = bring close to vesicle, also reognized by rab 7 in late endosome
hops = to lysosome, = conformational change so will bind to receptor compartment
to endosomes or to lysosomes
name endosome tethers
to endosomes = rab5 and corvet
to lysosomes = rab7 and hops
describe rab cascades
Rab5 at endosomes activates Rab7
Rab5 effector and tether (CORVET) is GEF for Rab7
Rab7 effector is GAP to inactivate Rab5
As Rab5 vesicles fuse with early endosome, more and more Rab7 is activated, less and less Rab5 stays active
Membrane becomes late endosome
describe clustering of tethers
Rab5 effectors have GEF or PI kinase activity on early endosome
GEF activity produces more Rab5-GTP in local area of membrane
PI-phosphates provide additional binding sites for vesicle tethers
Cluster of tethers form “landing site” for vesicle
describe the landing site for vesicle
type of microdomain that does not involve memebrane thickness or lipid content
not thickened but tether rich
describe snare proteins
family of membrane proteins that carry out vesicle fusion
what can rabs and tethers do
recruits snares to fusion site
membrane of vesicle and receptor compartment
describe vesicle snares and target snares - gen function
v snares on vesicles recognize t snares on target membranes
complexes form after tethering - complex provides specificity - where vesicle delivers cargo
unique combos of v and t snares determine targeting specificity
describe v snares
monomers with single tm helical domain
integral part
other proteins that interact with other proteins and 2 anchor proteins
describe t snares
trimers
combos of TM and peripheral subunits
describe what snares do
correct set of v and t snares form a stable tetramer
multiple snare complexes form at a target site to induce vesicle fusion
describe snare folding
v snare monomer = no stably folded, passively folded
t snare trimer = partially stable 2 hexlix bundle
v and t snare fold into v stable 4 helix bundle, produces bend and pulls close to target complex
folding process pulls membranes close together = becomes properly folded, generates physical strain like spring, not dependent on atp ot gtp, brings clsoe together and makes it fuse
describe fusion model - 4
- SNARE complexes form in a ring around the vesicle contact site
- The SNARE TM anchors are bent and strained, exerting a force that holds the membranes together
- The outer and inner layers of the membranes fuse
- The strain in the SNARE complex is relieved
describe snare dissosciation
After fusion, the SNARE complex is stable, unstrained and inactive
An AAA-family ATPase (NSF) dissociates v- and t-SNAREs – essential for continuation of vesicle traffic
t-SNAREs become active again
v-SNAREs are recycled back to their donor membrane by vesicles
describe NSF
NSF binds SNARE complex through adaptor protein (α-SNAP)
NSF twists and pulls during ATP hydrolysis
Multiple cycles of ATPase unwind the SNARE helices
describe endocytosis pathway early endosome – lysosome
CCVs mediate endocytosis from PM to early endosome
Some PM proteins are transported to recycling endosome, for exocytosis back to PM
lysosomal proteins are trafficked from trans-Golgi to endosomes, their receptors are trafficked back
Endosome membrane invaginates to form multivesicular body (MVB) and late endosome
Late endosome matures into lysosome, for degradation of proteins and lipids
describe receptor recycling - gen
like cholesterol = hydrophobic to protected by carrier
ldl receptors - clathrin coated put, endocytosis and uncoating and fusion with endosome into early endsome - lower ph so receptor unbinds carrier
fuses to lysosome
hydrolytic enzymes for carrier
returns ldl receptors to pm
frees cholesterol
describe receptor recycling - specific
Extracellular ligands are bound by transmembrane PM receptors, transported to early endosome for sorting
pH of early endosome is lower than the extracellular space, causing ligands to separate from receptors = changes the average charge on proteins, interactions are weakened
Empty receptors are recycled back to PM
Free ligands progress to lysosome
describe retrograde traffic
Retrograde traffic of proteins from endosomes to PM or trans-Golgi= extracellular receptors to PM, receptors that bring proteins to the lysosome are returned to Golgi
involve membrane tubules or tubular vesicle, not round coated vesicles
Requires the Retromer protein complex
does not depend on coated vesicles, but membrane tubules and retromer protein complex
describe endosome maturation - mono ub
Some TM proteins are marked for endocytosis by modification with mono-ubiquitin at PM=
– not poly-ubiquitin
– recognition by CCV adaptors
– if ubiquitin is removed, proteins are recycled to PM
– ubiquitin not removed is a signal for lysosomal degradation
tm proteins need to be degraded = if left then signals things, to degrade = early endosome makes another vesicle to grab receptor and bring inside vesicle, = multivesicular body, since cannot face cytosol
what turns into mvbs
early endosomes by invaginating and pinching off membrane
mvb contents cannot be recycled into pm anymore
describe mvb invagination
a series of ESCRT protein complexes shape and pinch off vesicles into lumen of endomsome
